The invention relates to a new method of producing epoxides via oxidation of olefins in a homogeneous gas-phase reaction by reacting ozone and NO2 and/or NO with the desired olefin under mild reaction conditions without using a catalyst. The method of the invention can be carried out as a continuous one-step method in a reactor according to FIG. 1, and requires very little technical input. Monoolefins having 2 to 16 carbon atoms and diolefins having 4 to 16 carbon atoms can be epoxidized.
Epoxides are important intermediates in the chemical industry and are mainly used in the production of olefin glycols or di-, poly- or oligomers thereof which mostly are processed further to form polyurethanes. In particular, propylene oxide and ethylene oxide each are required in amounts of about 5 million tons per year.
Epoxides can be produced from olefins by the chlorohydrin process, by indirect oxidation processes using peroxide reagents, and by catalytic or non-catalytic direct oxidation processes. The oxidations can be carried out in liquid phase or in gaseous phase. Oxidations in liquid phase, which can be carried out either as homogeneous or heterogeneous oxidations, are associated with difficult separation processes and complicated technologies. Direct olefin oxidation processes in the gaseous involve relatively long residence times and frequently furnish excessively low conversion and/or excessively low selectivity with respect to the epoxide.
Thus, for example, DE 197 54 303 A1 describes a method of producing propylene oxide from propylene in a homogeneous gas-phase reaction. While this method has a selectivity with respect to propylene oxide of  greater than 60%, the propylene conversion is relatively low, being 13% and 15%, respectively. In addition, the method is technically complex and costly because the reactor""s interior according to this invention is lined with an inert material, particularly with noble metals.
It was the object of the present invention to provide a method of producing epoxides by oxidizing olefins, which method requires low technical input, is favorable in cost, and ensures both high selectivity with respect to the epoxide produced and high conversion of the olefin employed.
It was found that epoxides can be produced via oxidation of olefins in a homogeneous, continuous gas-phase process with good yields and good selectivities with respect to the epoxide produced, by mixing ozone and NO2 and/or NO, optionally using a carrier gas, passing the resultant gas mixture into a conventional flow reactor, and reacting the corresponding olefin fed with the carrier gas under mild reaction conditions. According to the invention, a pressure of not more than 0.1-1000 mbar, preferably 1-500 mbar, and more preferably 1-200 mbar, is required. The temperature required is 50-350xc2x0 C., preferably 100-300xc2x0 C., and more preferably 140-240xc2x0 C. No catalyst is required in the method according to the invention. The residence time in the reaction zone is from 0.1 ms to a few seconds at maximum, and is preferably between 0.1 and 300 ms.
Oxygen, as well as inert gases such as helium, argon, nitrogen or mixtures thereof with oxygen can be used as carrier gases.
According to the invention, ozone and NO2 are employed at a ratio of  less than 0.5. Ozone and NO are preferably employed at a ratio of  less than 1.5.
In a preferred embodiment of the invention, ozone is supplied as an ozone/oxygen mixture, preferably 1-15 vol.-% ozone in oxygen, and more preferably 5-10 vol.-% ozone in oxygen.
The method of the invention is carried out in a conventional flow reactor supplied with said gas mixture of ozone and NO2 and/or NO and optional carrier gas. Preferably, the method is performed in a reactor according to FIG. 1.